Quasars are the most brilliant objects in the universe.
We can feel the Sun's heat from 150 million kilometers (93 million miles) away. Yet there are much hotter stars. Betelgeuse, a red supergiant in Orion, gives out 40,000 times as much energy as the Sun. One day its fuel will be gone and Betelgeuse will explode as a supernova. For a time - perhaps a year or more Ė it will shine as brightly as our whole galaxy.
A quasar is much brighter than a supernova. Its normal energy output is a hundred times greater than a galaxy of a hundred billion stars.
Despite their brightness, at first no one could see quasars.
Quasars were first discovered by radio telescopes in the 1950s, because some of them give off strong radio waves. No one knew exactly what the source of the radio waves was because they couldn't match them up with any visible object.
The first quasars seen by astronomers looked like weird stars.
Through a telescope, planets are disks, nebulae and galaxies are fuzzy, and stars look like points of light. When American astronomer Allan Sandage first saw a quasar, it was star-like, but its spectrum was really strange. A spectrum is the pattern of colors and lines you see when an objectís light is split up. The pattern tells astronomers what the object is made of and how hot it is. But this spectrum had lines in it that didnít match any known substance.
Quasars are not only incredibly bright, but also incredibly far away.
Dutch astronomer Maarten Schmidt first connected one of the unknown radio sources (3C 273) to one of the weird quasi-stellar (star-like) objects later called quasars. He realized that the strange lines in the spectrum were hydrogen, but that they were redshifted. This means that the lines had moved closer to the red end of the spectrum because the object was moving away from us.
You can see redshifted lines in this diagram. The bottom picture shows the spectral lines in the lab and the others show how they can be shifted. The amount of the shift tells us how fast the object is moving.
Once you know the velocity, you can calculate how far away the object is, and Schmidt was amazed to see that 3C 273 was two and a half billion light years away. Itís our closest quasar, but there are over 200,000 more known quasars, the farthest 13 billion light years away.
The big question was: What is out there thatís so bright that we can see it even from billions of light years away?
The big answer is: a supermassive black hole in the heart of a galaxy. A region about the size of the Solar System is responsible for all the activity, drowning out the light of the rest of the galaxy.
A supermassive black hole has the mass of a million suns or more, and itís surrounded by a disk of the material that it's pulling in. The material swirls round and round at high speed before falls into the black hole. This produces enormous amounts of radiation that we can detect, even though we canít see the black hole itself.
A telescope is a time machine and quasars are clues to the past.
Light travels at 300,000 kilometers (186,000 miles) per second. Thatís fast, but itís a big universe. We canít see anything until its light gets to us. So the light of a quasar five billion light years away takes five billion years to get here. Since Earth is 4.6 billion years old, we see the quasar as it was before Earth even existed.
We usually think of ourselves as surrounded by space that has objects in it which are farther and farther away to the edge of what we can see. It seems strange that there are no quasars within 2.5 billion light years, because after that they're common. We wonder why they're all so far away.
But instead, letís think of ourselves as looking back in time. When we see more distant things, we are seeing farther and farther into the past. That means that quasars belong to the history of the universe. Is there any evidence left of them nearby?
There are quasar-like objects and black holes in local galaxies.
Quasars are unusual because of the strength of the energy they give out. But similar objects are common much closer to us than 2.5 billion light years. We call them Active Galactic Nuclei (AGN). Powered by supermassive black holes, they also outshine their galaxies, only not as brightly as quasars.
In addition, itís likely that most galaxies contain a supermassive black hole. The Milky Way has one with a mass of four million Suns, but there isnít enough fuel for it to be active. Black holes don't suck things in - they can only affect matter that is nearby.
So it could be that the quasars became weaker as the nearby matter become scarcer, with most quieting down to leave only the black hole.
ďActive Galaxies and QuasarsĒ